1. Field of the Invention
The present invention relates to a projection lens system having a variable magnification function and a projector, and specifically relates, for example, to a projector using a highly monochromatic laser light source and an ideal projection zoom lens system for such a projector.
2. Description of Related Art
Zoom lens systems preferably usable as a projection lens system for projectors are proposed, for example, in Patent Literatures 1 and 2 listed below. Unfortunately, however, those zoom lens systems are designed for use with a white light source such as a xenon lamp, and thus they cannot be said to be suitable for a projector using a laser light source.
Patent Literature 1: JP-A-2010-008797
Patent Literature 2: JP-A-H10-161026
Laser light sources have an advantage that they are able to achieve a wide color reproduction range. To exploit the wide color reproduction range, it is necessary to preferably correct chromatic aberration in a wider wavelength range than conventional projection lens systems. Moreover, improvement has been sought in telecentricity for the purpose of achieving improved efficiency of combining colors by using a dichroic prism or of extracting illumination light by using a TIR (Total Internal Reflection) prism. There have been requests for a compact and low-cost zoom lens system that meets these requirements as a projection lens system.
With a white light source such as a xenon lamp, since light of each color has a somewhat wide wavelength range, even if chromatic aberration varies during zooming, the variation is averaged in the wavelength range, and this makes the chromatic aberration comparatively less noticeable. In contrast, with a single-color light source represented by a laser light source, since only a particular color is used, chromatic aberration is not averaged, and thus is noticeable.
In the zoom lens system described in Patent Literature 1, a third lens group is composed of three lens elements. In a compact projection lens system having this construction, lens groups would move by only a small moving amount in the vicinity of an aperture stop that corrects longitudinal chromatic aberration, and this makes it difficult to correct longitudinal chromatic aberration throughout the entire zoom range. And, if a sufficient moving amount were secured to correct longitudinal chromatic aberration, the size of the lens system would be increased and accordingly the cost would be increased.
In a zoom lens system disclosed in Patent Literature 2, part of movable lens groups from a third lens group onward move by a large moving amount. This increases variation in the passing position of an on-axis marginal light ray caused by zooming, and makes it difficult to reduce variation in longitudinal chromatic aberration caused by zooming.